Heretofore, it has been a common practice in the preparation of a honeycomb-structural body that a green body of ceramic mixture prepared by mixing ceramic material and molding aids and/or a pore-forming agent was molded firstly to a desired form in a forming mold and then extruded to obtain a desired honeycomb-structural body. The thus-prepared mold was fired in a continuous-feed furnace or in a periodic kiln under a predetermined temperature to attain an ultimate ceramic honeycomb-structural body.
In the firing of such a honeycomb-structural body, several admixtures are essentially employed to be mixed into the ceramic material, such as an organic binder, a surface-active agent (e.g., methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, starch paste, wheat flour, glycerol), a molding aids (e.g., wax and the like), and a pore-forming material (e.g., graphite, starch, sawdust and the like, all of which exhibit a differential specificity, as reviewed below. More particularly, the molding auxiliaries and pore-forming material may generally suffer from an inconvenience that they hardly can be burnt off (i.e., thermally decomposed) by heating from the outside of the honeycomb-structural body during the firing operation, while once they are burnt, they may be fired abruptly to generate heat. Additionally, each of such molding aids has a different ignition temperature. For this reason, there may possibly generate a substantial difference in the temperatures between the interior and the exterior of such a honeycomb-structural body. With such a differential temperature, it is possible that cracks may generate in the inside or end surface of the honeycomb-structural body or melting may occur in the interior thereof. Additionally, owing to a substantial difference in the degree of firing of such molding auxiliaries and/or pore-forming agent existing in the interior and the exterior of the honeycomb structure, the properties of such a honeycomb-structural body may possibly be non-uniform between the interior and the exterior of the structure. For instance, if a pore-forming agent in a certain area has burned abruptly, it generates irregularly greater pores in the specific area where there was such an abrupt burning. In this respect, therefore, it is generally the practice in the firing operation of such a ceramic green body to employ a smaller rate of temperature rising for the purpose of suppressing an abrupt firing in the interior of a ceramic green body in an attempt to prevent the generation of cracks and interior melting of such a green body and the resulting non-uniformity in the properties between the interior and the exterior of the fired products. However, with a lower temperature rising in the firing operation, a problem results in that the schedule of operation may naturally be extended, and the efficiency of production may be adversely affected.